Method and apparatus of physical property measurement using a probe-based nano-localized light source

ABSTRACT

An apparatus and method of performing physical property measurements on a sample with a probe-based metrology instrument employing a nano-confined light source is provided. In one embodiment, an SPM probe tip is configured to support an appropriate receiving element so as to provide a nano-localized light source that is able to efficiently and locally excite the sample on the nanoscale. Preferably, the separation between the tip apex and the sample during spectroscopic measurements is maintained at less than 10 nm, for example, using an AFM TR Mode control scheme.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.14/202,669, filed Mar. 10, 2014 and issued as U.S. Pat. No. 8,881,331,which claims priority under 35 USC §1.119(e) to U.S. Provisional PatentApplication Ser. No. 61/775,166, filed Mar. 8, 2013, both entitledMethod and Apparatus of Physical Property Measurement Using aProbe-Based Nano-Localized Light Source. The subject matter of thisapplication is hereby incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The preferred embodiments are directed to using a nano-localized lightsource to measure physical properties of a sample, and moreparticularly, to a method and apparatus of making nano-imaging andspectroscopy measurements using an atomic force microscope operating ineither contact or a low amplitude mode, with the tip apex of the probefunctioning as the nano-localized source.

2. Description of Related Art

The interaction between a sample under test and radiated energy can bemonitored to yield information concerning the sample. In spectroscopy,dispersion of light from a sample into its component energies can bemeasured and, for example, intensity plotted as a function ofwavelength. By performing this dissection and analysis of the dispersedlight, users can determine the physical properties of the sample, suchas temperature, mass and composition.

Notably, making spectroscopic measurements with a spatial resolution onthe nanoscale is continuing to improve. But, as noted by Berweger et al.in Adiabatic Tip-plasmon Focusing for Nano-Raman Spectroscopy, J. Phys.Chem. Letters (November 2010) (“Berweger 1”), the entirety of which ishereby incorporated by reference, despite ongoing progress in thedevelopment of imaging techniques with spatial resolution beyond thediffraction limit, simultaneous spectroscopic implementations deliveringchemical specificity and sensitivity on the molecular level haveremained challenging. Far-field localization techniques can achievespatial resolution down to about 20 nm by point-spread functionreconstruction, but typically rely on fluorescence from discretemolecular or quantum dot emitters, with limited chemically specificinformation. Scanning near-field optical microscopy (SNOM) providessub-diffraction-limited resolution through the use of tapered fibers orhollow waveguide tips. However, aperture-limited andwavelength-dependent fiber throughput reduces sensitivity, generallymaking SNOM unsuitable for spectroscopic techniques that have lowintrinsic signal levels.

In scattering-type SNOM (s-SNOM) external illumination of a sharp(metallic or semi-conducting) probe tip can enhance sensitivity,spectral range, and spatial resolution, as noted in Berweger. Chemicalspecificity can be obtained through the implementation of, for example,IR vibrational s-SNOM, tip-enhanced coherent anti-Stokes Ramanspectroscopy (CARS), or tip-enhanced Raman scattering (TERS). Here theantenna or plasmon resonances of the (noble) metal tips can provide thenecessary field enhancement for even single-molecule sensitivity.

In the standard implementation, however, the direct illumination of thetip apex results in a three-to-four orders of magnitude loss inexcitation efficiency, related to the mode mismatch between thediffraction-limited far-field excitation focus and the desired tens ofnanometers near-field localization, as determined by the tip apexradius. The resulting loss of sensitivity, together with a far-fieldbackground signal, often limit contrast and may cause imaging artifacts,presenting challenges for the general implementation of a wider range ofspectroscopic techniques in s-SNOM.

A general solution for optical nano-imaging and spectroscopy thusrequires a true nano-localized light source. While this can be achievedthrough a nanoscopic emitter in the form of a single molecule, quantumdot, or nanostructure at the apex of a tip, that approach relies on thequantum efficiency and spectral characteristics of the emitter, and thedifficulties of overcoming the intrinsic background and sensitivitylimitations with unmatched far-field mode excitation remain.

In one known technique, a nanoemitter is generated through nonlocalexcitation, taking advantage of the effective tip comic radius-dependentindex of refraction n(r) experienced by a surface plasmon polariton(SPP) propagating along the shaft of a noble metal tip. The resultingpropagation-induced adiabatic SPP focusing into the tip apex region isdue to the continuous transformation of the surface mode size. Thisapproach allows for SPP coupling spatially separated from the tip apex,and subsequent probe apex excitation via the propagating SPP, with tensof nanometers field confinement over a broad spectral range with highfocusing efficiency.

As further noted in Berweger, the use of a photonic crystalmicroresonator as a coupling element on a tip has previously been usedto demonstrate TERS. As explained by the authors, it was believed thatthe geometric constraints of the cantilever-based design made the studyof opaque samples difficult, and the collinear excitation and residualhole-array transmission did not yet fully eliminate the far-fieldbackground. In a known analogous but simplified approach, agrating-coupler was employed to launch SPP modes onto the shaft ofmonolithic gold (Au) tips. The conical tips with two-stage optical modematching of the far-field SPP coupling and the mechanisms of adiabaticSPP field concentration into the tip apex represent a unique opticalantenna concept for the efficient far-field transduction into nanoscaleexcitation.

As shown in FIG. 1, the nanofocusing process can be implemented byemploying an etched tip and using side-illumination grating coupling. Aschematic image of an electrochemically etched Au probe tip 200 withplasmonic grating 206 is shown. An optical image 208 is superimposed onthe illustration showing the illumination of grating 206 with afar-field focus for launching SPPs, which then propagate non-radiativelyalong the shaft or body 202 of tip 200, with corresponding localizedemission from the nanofocused field at the tip apex 204 (the energy iscollected and concentrated at the apex as a localized source,represented schematically with a dimension “d”). The apex-emitted lightfollows a cos² (θ) polarization dependence, as expected for a nanoscopicdipolar emitter located at the tip apex. These results confirm theexpected mode filtering of the nanofocusing process, as only theradially symmetric m=0 SPP mode will produce purely axial dipoleemission, due to destructive interference of the radial polarizationcomponents.

One promising technology for improving spectroscopic measurementperformance is scanning probe microscopy. Scanning probe microscopes(SPMs), such as the atomic force microscope (AFM), are devices whichtypically employ a probe having a tip and causing the tip to interactwith the surface of a sample with appropriate forces to characterize thesurface down to atomic dimensions. Generally, the probe is introduced toa surface of a sample to detect changes in the characteristics of asample. By providing relative scanning movement between the tip and thesample, surface characteristic data can be acquired over a particularregion of the sample and a corresponding map of the sample can begenerated.

A typical AFM system is shown schematically in FIG. 2. An AFM 10employing a probe device 12 including a probe 14 having a cantilever 15.Scanner 24 generates relative motion between the probe 14 and sample 22while the probe-sample interaction is measured. In this way images orother measurements of the sample can be obtained. Scanner 24 istypically comprised of one or more actuators that usually generatemotion in three orthogonal directions (XYZ). Often, scanner 24 is asingle integrated unit that includes one or more actuators to moveeither the sample or the probe in all three axes, for example, apiezoelectric tube actuator. Alternatively, the scanner may be anassembly of multiple separate actuators. Some AFMs separate the scannerinto multiple components, for example an XY scanner that moves thesample and a separate Z-actuator that moves the probe. The instrument isthus capable of creating relative motion between the probe and thesample while measuring the topography or some other surface property ofthe sample as described, e.g., in Hansma et al. U.S. Pat. No. RE 34,489;Elings et al. U.S. Pat. No. 5,266,801; and Elings et al. U.S. Pat. No.5,412,980.

In a common configuration, probe 14 is often coupled to an oscillatingactuator or drive 16 that is used to drive probe 14 at or near aresonant frequency of cantilever 15. Alternative arrangements measurethe deflection, torsion, or other motion of cantilever 15. Probe 14 isoften a microfabricated cantilever with an integrated tip 17.

Commonly, an electronic signal is applied from an AC signal source 18under control of an SPM controller 20 to cause actuator 16 foralternatively scanner 24) to drive the probe 14 to oscillate. Theprobe-sample interaction is typically controlled via feedback bycontroller 20. Notably, the actuator 16 may be coupled to the scanner 24and probe 14, but may be formed integrally with the cantilever 15 ofprobe 14 as part of a self-actuated cantilever/probe.

Often, a selected probe 14 is oscillated and brought into contact withsample 22 as sample characteristics are monitored by detecting changesin one or more characteristics of the oscillation of probe 14, asdescribed above. In this regard, a deflection detection apparatus 25 istypically employed to direct a beam towards the backside of probe 14,the beam then being reflected towards a detector 26. As the beamtranslates across detector 26, appropriate signals are transmitted tocontroller 20, which processes the signals to determine changes in theoscillation of probe 14. In general, controller 20 generates controlsignals to maintain a relative constant interaction between the tip andsample (or deflection of the lever 15), typically to maintain a setpointcharacteristic of the oscillation of probe 14. For example, controller20 is often used to maintain the oscillation amplitude at a setpointvalue, A_(S), to insure a generally constant force between the tip andsample. Alternatively, a setpoint phase or frequency may be used.

A workstation is also provided, in the controller 20, and/or in aseparate controller or system of connected or stand-alone controllers,that receives the collected data from the controller and manipulates thedata obtained during scanning to perform point selection, curve fitting,and distance determining operations.

AFMs may be designed to operate in a variety of modes, including contactmode and oscillating mode. Operation is accomplished by moving eitherthe sample or the probe assembly up and down relatively perpendicular tothe surface of the sample in response to a deflection of the cantileverof the probe assembly as it is scanned across the surface. Scanningtypically occurs in an “x-y” plane that is at least generally parallelto the surface of the sample, and the vertical movement occurs in the“z” direction that is perpendicular to the x-y plane. Note that manysamples have roughness, curvature and tilt that deviate from a flatplane, hence the use of the term “generally parallel.” In this way, thedata associated with this vertical motion can be stored and then used toconstruct an image of the sample surface corresponding to the samplecharacteristic being measured, e.g., surface topography. In one mode ofAFM operation, known as TappingMode™ AFM (TappingMode™ is a trademark ofthe present assignee), the tip is oscillated at or near a resonantfrequency of the associated cantilever of the probe. A feedback loopattempts to keep the amplitude of this oscillation constant to minimizethe “tracking force,” i.e. the force resulting from tip/sampleinteraction. Alternative feedback arrangements keep the phase oroscillation frequency constant. As in contact mode, these feedbacksignals are then collected, stored, and used as data to characterize thesample. Note that “SPM” and the acronyms for the specific types of SPMs,may be used herein to refer to either the microscope apparatus or theassociated technique, e.g., “atomic force microscopy.”

Regardless of their mode of operation. AFMs can obtain resolution downto the atomic level on a wide variety of insulating or conductivesurfaces in air, liquid, or vacuum, by using piezoelectric scanners,optical lever deflection detectors, and very small cantileversfabricated using photolithographic techniques. Because of theirresolution and versatility, AFMs are important measurement devices inmany diverse fields ranging from semiconductor manufacturing tobiological research.

In one embodiment employing an AFM for optical excitation and detection,the system provides side-on illumination of the tip shaft (body) of atip, supporting a grating. The electrochemically etched tips may bemounted onto an AFM quartz tuning fork, and the gating is fabricated viafocused ion beam (FIB) milling. But such known systems have limitations.For example, the tip-sample coupling relationship is complex and can,for example, result in a loss of excitation enhancement due to the broadk-vector distribution at the apex allowing for launching propagatingSPPs at the sample surface. This is suggested by reduced fundamentalapex emission often observed upon approach of an Au tip surface. Also,use of electrochemically etched tips is limited to shear forcemicroscopy (tuning fork tips—difficult to implement tip-samplemodulation with these). As a result, the AFM modes available forsimultaneous and sensitive topographic, phase, and other state of theart AFM functions is severely limited. Electrochemically etched tips arealso difficult to mass produce, and their reproducibility is limited. Inaddition, the materials that can be used to produce the tips arelimited. Electrochemical etching allows for the fabrication of sharptips with only certain metals, such as gold, silver, or tungsten. Andwith large amplitude AFM modes, the tip is removed from the opticalnear-field during spectral acquisition, thus producing a weakerspectroscopic signal as the tip on average is further away from thesample.

As a result, improvements were needed to expand the range and efficiencyof performing optical nanoimaging and spectroscopy on the nanoscale.

SUMMARY OF THE INVENTION

Efficiency of optically making physical property measurements on asample is facilitated by employing a combination of an atomic forcemicroscope (AFM) capable of maintaining a tip-sample separationsufficiently small with SPM tips designed with a unique geometry (e.g.,waveguide) that can be used to transform a quasi-planar SPP mode coupledvia a receiving element into a spatially confined excitation source. Ina preferred embodiment, the so-called torsional resonance mode (TR Mode)may be used to maintain tip-sample separation less than 100 nm, thoughTapping Mode AFM using low amplitude oscillation may also be employed.Combined with the described antenna concept, the preferred embodimentsprovide highly efficient generation of nano-localized excitations withan inherently broad operational bandwidth and only a weak wavelengthdependence. In a scanning probe microscope, this allows for nearlybackground-free s-SNOM imaging with any spectroscopic techniques, andassociated chemical specificity. The preferred embodiments are usable inspectroscopy set-ups, as well as in nano-imaging applications employinga set wavelength.

In the present preferred embodiments, an SPM probe includes a cantileverand a tip, with the tip positioned at the distal end of the cantilever,and including a) a shaft and, b) an apex positioned adjacent to asample. A receiving element is provided and supported by the shaft ofthe tip. A source of electromagnetic wave excitation directselectromagnetic waves toward the receiving element, the electromagneticwave being coupled to the apex via the receiving element. The coupledelectromagnetic excitation at the apex is locally enhanced and interactswith the sample. A controller maintains a separation between the apexand the sample greater than zero nanometers and less than 100 nm duringelectromagnetic wave excitation.

In accordance with a further aspect of the preferred embodiments, thecontroller maintains the separation using torsional resonance mode (TRMode) feedback.

According to another aspect of the preferred embodiments, the controllermaintains the separation at less than 5 nm using tapping mode, wherein atapping mode setpoint amplitude is between about 0.1 nm and 10 nm.

In another aspect of the preferred embodiments, the shaft has acontinuous surface around its entire periphery, and the continuoussurface is at least one of a conical surface and an elliptical surface.A pyramid shape may alternatively be employed.

In a further aspect of the preferred embodiments, the receiving elementis supported by the tip, and is one of a surface grating, a prism, aphotonic crystal, a waveguide, and an optical antenna, or any otherstructure or material suitable to achieve the coupling of the incidentelectromagnetic wave which yields the excitation of the apex.

According to another aspect of the preferred embodiments, the apexcomprises a conductive metal, or any other material suitable to interactwith the electromagnetic wave in order to achieve the apex excitation,such as a highly doped semiconductor, and has a radius between about 1and 100 nm.

Further, the source of electromagnetic wave excitation is one of a) alaser operated at a wavelength in the UV to near-IR spectral range(about 300 to 1200 nm) (e.g., to induce a Raman shift), and b) an IRsource operating at a wavelength equal to about 2-30 μm.

In another aspect of the preferred embodiments, the locally enhancedsignals are used in at least one of a spectroscopic measurement and anano-imaging measurement.

A method configured in accordance with the preferred embodimentsincludes optically measuring a physical property of a sample byproviding an AFM including a probe having a cantilever and a tipsupported at about a distal end of the cantilever, the tip including ashaft and an apex. The method also includes providing a receivingelement supported by the shaft of the tip, and a source ofelectromagnetic wave excitation. Next, electromagnetic waves aredirected from the source toward the receiving element, and theelectromagnetic waves are coupled from the receiving element to the apexto produce locally enhanced fields that interact, with the sample.Finally, the method includes controlling a separation between the apexand the sample to be greater than zero nanometers and less than 100 nmduring electromagnetic wave excitation.

In another aspect of the preferred embodiments, the controlling stepmaintains the separation using at least one of a) torsional resonancemode (TR Mode) feedback, b) tapping mode feedback, wherein a setpointamplitude associated with the tapping mode feedback is between about 0.1nm and 10 nm, and e) contact mode feedback.

According to another aspect of the preferred embodiments, the receivingelement provides adiabatic plasmon focusing to produce the locallyenhanced signals and is at least one of a surface grating, a prism, aphotonic crystal, a waveguide, and an optical antenna.

In another aspect of the preferred embodiments, the controlling stepmaintains the separation at less than 5 nm using at least one oftorsional resonance mode (TR Mode) and tapping mode operated at asetpoint amplitude between about 0.1 nm and 10 nm.

In another preferred embodiment, an SPM includes a probe including acantilever with a tip, the tip having a shaft and an apex. The SPM alsoincludes a receiving element supported by the probe, and a remote sourceof electromagnetic wave excitation that directs electromagnetic wavestoward the receiving element, the receiving element providing adiabaticplasmon focusing of the electromagnetic waves to excite the apex andproduce locally enhanced excitation that is coupled to a sample, thelocally enhanced excitation being comprised entirely of the focusedelectromagnetic waves. A controller maintains a separation between theapex and the sample greater than zero nanometers and less than 100 nm.

According to another embodiment, an SPM includes a probe having acantilever and a tip, the tip positioned at a distal end of thecantilever and including a) a shaft and, b) an apex positioned adjacentto a sample. A receiving element supported by the shaft of the tip isalso provided. The SPM next includes a source of electromagnetic waveexcitation that directs electromagnetic waves toward the receivingelement, the electromagnetic waves being coupled to the apex via thereceiving element, and wherein the coupled electromagnetic waves at theapex yield locally enhanced, background free spectroscopic signal thatinteracts with the sample.

In another aspect of the preferred embodiments, a method for opticallymeasuring a physical property of a sample, the method includes providinga scanning probe microscope (SPM) including a probe having a cantileverand a tip supported at about a distal end of the cantilever, the tipincluding a shaft and an apex. The method also includes providing ananostructure at the apex of the tip. The nanostructure is illuminatedand thereby optically excited so as to produce a locally enhancedspectroscopic signal that interacts with the sample. As a result, themethod is able to measure a property of the sample based on theinteraction.

These and other features and advantages of the invention will becomeapparent to those skilled in the art from the following detaileddescription and the accompanying drawings. It should be understood,however, that the detailed description and specific examples, whileindicating preferred embodiments of the present invention, are given byway of illustration and not of limitation. Many changes andmodifications may be made within the scope of the present inventionwithout departing from the spirit thereof, and the invention includesall such modifications.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred exemplary embodiments of the invention are illustrated in theaccompanying drawings in which like reference numerals represent likeparts throughout, and in which:

FIG. 1 is a schematic illustration of a nano-localized light source ofthe Prior Art;

FIG. 2 is a schematic illustration of a Prior Art atomic forcemicroscope AFM;

FIGS. 3 and 4 are schematic illustrations of a system for convertingfar-field radiation to near field excitation, with light input (FIG. 3)and electrical power input (FIG. 4);

FIG. 5 is a schematic perspective illustration of a nano-localized lightsource generated at the apex of a tip of an AFM cantilever probe,according to a preferred embodiment;

FIG. 6 is a schematic illustration of the nano-localized light source ofFIG. 5, showing a grating-type receiving element;

FIG. 7 is a schematic perspective illustration of the nano-localizedlight source of FIG. 6, employed in an AFM operating in torsionalresonance mode (TR Mode).

FIG. 8 is a schematic block diagram of a TR Mode AFM employing thenano-localized light source of FIG. 6;

FIG. 9 is an AFM set-up using a nano-localized light source, includingultrafast pulse shaping and broadband grating nanofocusing.

FIG. 10A is a plot of a dispersion relationship of a localized plasmonresonance (optical antenna) and an ideal point dipole;

FIG. 10B is a schematic illustration of the near-field dipole-dipolecoupling between the antenna and nanoscale quantum system, for example;

FIG. 11A-11C are schematic illustrations of probe shapes usable in thepreferred embodiments;

FIG. 12 is a plot of the dispersion relationship for SPPs propagating ona cylindrical waveguide;

FIG. 13 is a plot corresponding to FIG. 12 of a normalized radialelectric field at different locations;

FIG. 14 is a plot of elastic scattered light as the AFM probe progressesover a sharp step edge of the sample, also illustrating the sampletopography and the elastic scattered light when alternatively directlyilluminating the tip apex, appropriately labeled Prior Art; and

FIG. 15 is a plot illustrating the spatio-temporal regime of electronicand vibrational excitations in a sample, showing time resolution down toa single cycle with ultrafast optical microscopies, constrained by thediffraction limit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An improved apparatus and method of performing nano-image andspectroscopic measurements efficiently and locally on the nanoscale isshown and described. In one embodiment, sharp tipped probes employed ina conventional metrology instrument, such as an atomic force microscope(AFM), are configured to provide a top-down nano-localized light source.Rather than using far-field light which has a low density of states(essentially only one mode), the nano-localized light source of thepreferred embodiments provides a localized optical dipole of highintensity. The concept is akin to Fórster resonance energy transfer(FRET), which looks at the energy transfer from one excited dye molecule(chromophore) to another to measure distances between the molecules. Inanalogous fashion, the nano-localized light source employed in thepreferred embodiments is used as on an SPM probe to provide highlyefficient excitation of the sample, thus allowing the preferredembodiments to make spectroscopic measurements efficiently and on ahighly localized scale. Efficiency is further facilitated by preferablymaintaining close separation between probe apex and the sample byoperating the AFM in a low amplitude mode, such as torsional resonancemode (TR Mode), contact mode or Tapping Mode with an amplitude betweenabout 0.1 nm and 10 nm.

Most generally, turning to FIGS. 3 and 4, the preferred embodiments areable to take a standard input (e.g., electromagnetic wave excitationsuch as illumination) and bring the associated energy to the near fieldfor localized spectroscopy. In the system of FIG. 3, far-fieldelectromagnetic radiation 42 having a wave number 2π/λ is input to aFar-to Near-Field transformation block 43. The near-field radiation canthen be received by an appropriate receiving element 44, such as amolecule, nano structure, quantum system, etc., for exciting acorresponding spectroscopic measurement. The scattered near-field energyis collected by element 45 and transmitted to an associated Near- toFar-field transformation block 46 that outputs far-field energy 47 forprocessing, i.e., ultimate detection and analysis of sample properties.

In the system of FIG. 4, electrical energy (J/V) is used as the input.This energy is input to for example, a light emitter 48 that transmitselectromagnetic energy to antennas for transmission to a near fieldreceiver 49 for conversion of the energy to the near-field. Thenear-field receiver, similar to the system of FIG. 3, can be a molecule,nano structure, quantum system, etc. for performing localizedspectroscopy.

A schematic illustration of a preferred implementation of such a system,including the production of a nano-localized source, is provided in FIG.5. In this case, the probe based optical antenna shown in FIG. 1 isemployed as part of a AFM-type cantilever probe 50. More particularly, atip 54 of probe 50 is coupled to a distal end 53 of a cantilever 52 ofprobe 50 and operates as the antenna (nano-localized source). Tip 54includes a shaft 56 and an apex 58, with shaft 56 supporting a receivingelement 60. In this way, in contrast to known systems that employ tuningfork probes (shear force microscopy), AFM functions, such as measuringtopography and phase, can be performed with the same instrument,simultaneously.

The preferred embodiments provide side-on illumination (Far Field) oftip shaft (body) 56, with shaft 56 supporting receiving, element 60,such as a surface grating, a prism, a photonic crystal, a waveguide, oran optical antenna. Receiving element 60 facilitates the collection ofthe electromagnetic waves which propagate down shaft 56 of tip 54 towardapex 58, i.e., launch of a surface plasma polariton (SPP). Thenano-localized source is realized at apex 58 with no direct illuminationof the sample, i.e., the localized near-field energy that interacts withthe sample can be generated entirely with indirect illumination of theprobe. The corresponding Near-Field interacts with a sample 61 beingexamined, and as this Near-Field interacts with sample 61, the scatteredlight (“Scattered near-field”) is detected, for example, using 90°sagittal detection with detector 62, as shown schematically in FIG. 5.

A schematic illustration of a gold (Au) tip 54 with gratine, 60′supported by shaft 56 is shown in FIG. 6, superimposed with a far-fieldoptical image 70 of grating illumination upon excitation by far fieldillumination, “L.” The production of the nano-localized source includesthe effect of grating-coupling, SPP propagation, and ultimatelynano-localized tip apex emission. Positioning the AFM probe 50 adjacentto a surface 74 of a sample 72 allows the nano-localized source tointeract with the sample so spectroscopic and nano-imaging measurementsmay be made. The distance “s” between apex 58 and sample surface 74 ispreferably maintained at less than 100 nm to insure efficient couplingand maximize resolution. Low-amplitude AFM modes are used to achievethis result, as described further below.

In FIGS. 7 and 8, a system set-up for one preferred mode of AFMoperation to maintain tip-sample separation at less than 100 nm isillustrated. In torsional resonance mode (TR Mode), a probe assembly 100includes a probe 101 and a base 102. Probe 101 includes a cantilever 104and a tip 106 supported at the distal end of lever 104 and having anapex 107. In this case, tip 106 includes a receiving element 108 (e.g.,a grating), toward which electromagnetic wave excitation “L” isdirected.

In TR Mode, at least the tip 106, and preferably the entire cantilevermotion of probe 101 is initially driven into oscillation at or near atorsional resonance of the probe using any of the techniques describedin U.S. Pat. Nos. 6,945,099 and 7,168,301, owned by the presentapplicant, the entirety of each of which is expressly incorporated byreference herein. The motion of the tip is as schematically shown(phantom), which is rotation substantially about longitudinal axis A′ ofcantilever 104. The separation between tip 106 and sample 110 (FIG. 8)is then reduced (e.g., by exciting actuator 114 in Z) to cause the twoto interact. A beam of light “B” generated by probe oscillationdetection system 116 is directed towards a back of cantilever 104 sothat it is reflected therefrom. The reflected beam is then sensed by adetector 118. Preferably, detector 118 is a quadrature (i.e., four-cell)photodetector.

With further reference to FIG. 8, torsional resonance control isillustrated. Torsional oscillation is detected by detector 118, whichoutputs signals indicative of interaction between probe tip 107 andsample surface 112. A signal processing block 120 conditions the probemotion signals for comparison to a set-point associated with TR Modeoscillation in block 122. An appropriate gain is applied by block 124 tocommand appropriate motion of actuator 114 supporting sample 110 tomaintain probe oscillation at the TR Mode set-point. This low-amplitudemode of AFM operation is effective at maintaining tip-sample separationat the desired distance (e.g., less than 100 nm), important to insuringgood coupling between the enhanced optical excitation emitted at the tipduring the above-described electromagnetic excitation.

As an alternative, tapping mode AFM may be employed. In this case, lowamplitude oscillation should be maintained so tip-sample separation ismaintained at less than 100 nm. Preferably, the amplitude of the tappingmode drive should be set to be within a range of about 0.1 nm and 10 nm.Finally, in contact mode, the tip and sample remain “in contact,” asunderstood in the art. This insures tip-sample separation is maintainedwithin the 100 nm parameter yielding superior spectroscopic signals, andnano-scale resolution. Ultimately, data quality is improved, givengreater sensitivity compared to simply illuminating the probe apex, asin previous systems.

Turning to FIG. 9, plasmonic nanofocusing in combination withfrequency-domain pulse shaping allows for the generation of ultrashortpulses in the nano-localized region of the tip apex, as illustrated. Anano-localized source 150 includes an originating pulsed laser 152(Ti:Sa) that outputs radiation to a pulse shaper 154 that conditions theradiation prior to focusing by a lens 156. The output of lens 156 isdirected generally orthogonally to the shaft or body 160 of a tip 158extending from the distal end of a cantilever 157 of the AFM probe. Agrating 162 disposed on shaft 160 facilitates generation of the SPY 164toward the tip apex 166.

Full characterization and control of the nanofocused pulses is achievedthrough the local second-harmonic generation (SHG) resulting from thebroken axial symmetry of the tip apex, providing access to the spectralphase at this point. A nanofocused pulse measured at the tip-apex withits intensity and phase reconstructed from interferometric frequencyresolved optical gating (FROG). The temporal duration of the pulse witha spectral width of ˜60 nm is found to be ˜16 fs and transform-limitedafter optimization using a multi-photon intrapulse interference phasescan algorithm (MIIIPS).

With the nanofocusing mechanism being to first order independent ofwavelength and spectral phase, it not only allows for achieving theshortest possible pulse duration, but also for generation of arbitraryoptical waveforms at the apex through deterministic pulse shaping, onlylimited by the spectral bandwidth at the apex. For tips exhibiting alocalized plasmon resonance at the apex near the laser frequencies,plasmon dephasing times of T₂≅20 fs are expected to provide a lowerlimit for the achievable apex pulse duration. For non-resonant tips, theshortest attainable nanofocused pulse duration is determined by thespectral bandwidth available.

With more specific reference to the characteristics of probe 50, shaft56 preferably has a continuous surface around its entire periphery toinsure efficient propagation of the SPP. In this regard, thecross-section of the shaft is preferably conical or elliptical.Receiving element 60 (FIG. 5) can either have dimension (like a prism)or comprise a grating, as discussed previously. The radius of apex 58 ispreferably in the range of 1 nm to 100 nm. While the composition of theprobe may have a variety of characteristics, the apex material isimportant. Apex 58 is preferably made of a conductive metal, or ofsilicon with a metal coating, such as gold (Au), silver (Ag), aluminum(Al), or other suitable metal. Electromagnetic wave excitation (e.g.,“L” in FIG. 6) can be a laser with an excitation wavelength for Ramanshift, or an IR laser having a wavelength range of about 2-30 μm.

The tip-scattered Raman light generated by the localized apex plasmonmay be spectrally filtered and detected using a grating spectrometer.Confocal spatial filtering of the apex emission prevents residualgrating-scattered light from reaching the spectrometer. Analytemolecules may be deposited by spin coating from solution onto anevaporated Au surface, providing additional field enhancement fromplasmonic tip-sample coupling.

As noted in Berweger I, the tip cone angle of ˜15° corresponds to amaximum nanofocusing efficiency at ≅800 nm, yet with broad wave-lengthrange.

The side-on illumination geometry (rather than under sampleillumination) allows the surfaces of thick, bulk and nontransparentsamples to be studied. Furthermore, using AFM with interaction forces onthe order of 10's of pN (for example, low amplitude tapping mode AFM)provides a reduced force perturbation of the sample by several orders ofmagnitude. Notably, the grating 60′ may be fabricated via focused ionbeam (FIB) milling, with the grating period ao determined by thein-plane momentum conservation condition k_(SPP)=k_(in)+nG, with integern and G=2πlao.

Notably, although comparable near-field signal levels for both thegrating illumination TERS and direct apex illumination TERS are found,the background signal, its origin, and the resulting contrast arefundamentally different. For direct apex illumination, the residualbackground of the TERS signal (with the tip retracted) originates fromthe elliptical diffraction-limited far-field focus with major and minorradii of ˜2 and 1 μm, respectively. In contrast, for gratingillumination of the same tip, no far-field. Raman background is observedas a result of the intrinsic nanometer-scale spatial field confinementachieved by the SPP propagation-induced nanofocusing at the apex.

Generation of Nano-Localized Source (Conical Antenna)

As noted in Berweger et al. Light on the Tip of a Needle: PlasinonicNanofocusing for Spectroscopy on the Nanoscale, J. Phys. Chem. Letters(March 2012) (hereinafter “Berweger II”), the entirety of which isincorporated by reference herein, efficient, reproducible, scalable, andtunable focusing of light into the nanoscale for enhanced spectroscopyand imaging has remained challenging. While in principle thediffraction-limited size of a far-field focus is well matched to theabsorption cross-section of an ideal resonant dipolar quantum absorberof 3ë2/2

, molecular cross-sections are typically limited by non-radiativedecoherence via intra- and inter-molecular coupling to approximately thegeometric size of the molecule. Overcoming this mode mismatch is thegoal of using optical antennas, which have evolved from the uncontrolledyet high electromagnetic field enhancement in rough SERS substrates, tocontrollable, yet often still inefficient antenna devices to mediate anddrive the interaction between free-space light and molecular ornanoscale excitations.

Optical antennas are conceptually analogous to RF antennas: first, apropagating far-field wave is absorbed by the antenna and transducedinto a wire-bound and guided electrical current. Second, the antennacurrent at RF frequencies is then transferred to and converted by theload. Extension of this concept to optical frequencies is complicated bylow electrical conductivities leading to large ohmic losses, and thelack of analogous discrete circuit elements preventing optimal impedancematched conditions.

Construction of an optical antenna in this regard preferably employs theuse of noble metal nanoparticles exhibiting strong polarizabilities atoptical frequencies arising from localized (electronic) surface plasmonpolariton (SPP) resonances. As illustrated in FIG. 10A, this inprinciple enables the transformation of far-field modes with a singlefree-space wavevector k₀=2πλ₀=ω₀/c into spatially localized evanescentmodes with correspondingly large wavevector distributions (red). Throughthe mode overlap of the large wavevector distribution with that of theinduced molecular point dipole as the load, electromagnetic energytransfer occurs via evanescent dipole-dipole coupling as illustrated inFIG. 10B. The efficiency of this process then depends on theantenna-to-load separation R analogous to e.g., Förster resonant energytransfer (FRET) between two chromophores, with the transfer efficiency1/(1+(R/R₀)⁶) and typical values of the Förster distance R₀ of a fewnanometers, depending on spectral overlap and relative orientation. Viareciprocity the reverse process is also facilitated, i.e., enhancedradiative antenna-coupled molecular emission.

Due to the high field localization and enhancement generally enabled byplasmon resonances, optical antennas have found widespread applicationsfor sensing and spectroscopy, most notably in surface enhancedspectroscopies such as Raman scattering (SERS), infrared absorption(SEIRA), and coherent nonlinear optics such as second-harmonicgeneration (SHG). All-optical imaging with nanometer spatial resolutionis enabled by scattering-type scanning near-field optical microscopy(s-SNOM), and its plasmon-enhanced variant tip-enhanced Raman scattering(TERS) can yield single molecule sensitivity. However, despite muchprogress, significant difficulties had persisted for providing ageneralizable nanofocusing concept.

While the absorption cross section (effective area) of plasmonicnanoparticles can approach the theoretical limit of RF dipole antennasof óRF,dip=0.13ë2, increasing ó beyond the dipole limit by, for example,increasing the volume of the plasmonic structure, results in reducedspatial field localization due to smaller associated wavevectors. Theintrinsically low Q-factors (on the order of 10) arising from shortplasmon dephasing times T2, while providing broad bandwidth, limit thelocal field enhancement FαT2. In order to overcome the limited crosssections of single or simple coupled plasmonic nanostructures, gradualmode transformations based on cascaded structures and focusingpropagating SPP modes into structural singularities such as wedges andgrooves have been proposed, as well as the extension of classicalantenna concepts such as coplanar strip lines and phased array antennas,as illustrated in FIGS. 11A-11C. While improved performance can beachieved with gradual or multi-step mode transformations, the use ofresonant structures and nanoparticles remains limited in efficiency dueto scattering and radiation losses at the structural discontinuities andcoherent nonlinear optics such as second-harmonic generation (SHG).All-optical imaging with nanometer spatial resolution is enabled byscattering-type scanning near-field optical microscopy (s-SNOM), and itsplasmon-enhanced variant tip-enhanced Raman scattering (TERS) can yieldsingle molecule sensitivity. However, despite much progress, significantdifficulties had persisted for lack of a generalizable nanofocusingconcept. (see Berweger II)

As discussed in Berweger I and II, and elsewhere, in using AFM toperform the spectroscopic measurements, the probe is configured as a 3Dregularly tapered conical waveguide, thereby providing a nano-localizedlight source and overcoming the limitations described with respect tothe art. Its unique topology possesses no structural discontinuitiesexcept at the apex, thus minimizing all scattering losses. This allowsfor a continuous surface plasmon polariton (SPP) mode transformationtaking advantage of a radius-dependent effective index of refraction(neff(r)∝1/r for r<<λ0) experienced by SPP's propagating on the outsidesurface of the structure. The increasing index of refraction leads to adecreasing SPP wavelength, thus avoiding scattering loss as the tapernarrows, allowing efficient generation of a spatially localizedexcitation at the waveguide output or terminus.

In general, the dispersion relationship of an SPP is kSPP=k₀n_(eff),with n_(eff) for the case of a planar or large cylindrical materialinterface given by Equation (1),

${n_{eff} = \sqrt{\frac{ɛ_{1}ɛ_{2}}{ɛ_{1} + ɛ_{2}}}},$with dielectric function of the metal and surrounding dielectric, ∈₁ and∈₂, respectively. For Re(∈₁)<0 and |Re(∈₁)|>1, as is the case for noblemetals over abroad frequency range up to the visible spectrum, kSPP>k₀;i.e. momentum conservation prevents the coupling of SPP's to free-spacelight and the SPP wave remains surface confined. However, since Im(∈₁)>0, Im(kSPP)>0 results in appreciable ohmic loss, and thus, SPPpropagation attenuation.

For a radially symmetric (m=0) mode propagating on a cylindricalwaveguide of radius r, n_(eff)(r) can be calculated by solving thetranscendental equation (Equation (2)),

${{\frac{ɛ_{1}}{\kappa_{1}}\frac{I_{1}\left( {k_{0}\kappa_{1}r} \right)}{I_{0}\left( {k_{0}\kappa_{1}r} \right)}} + {\frac{ɛ_{2}}{\kappa_{2}}\frac{K_{1}\left( {k_{0}\kappa_{2}r} \right)}{K_{0}\left( {k_{0}\kappa_{2}r} \right)}}} = 0$with the modified Bessel functions Ij and Kj (j=0,1) andκi=√n2_(eff)(r)−∈i. While higher order (m=1, 2, 3 . . . ) asymmetricmode solutions also exist, they do not experience the divergingn_(eff)(r) with decreasing “r” necessary for nanofocusing. Instead thesemodes have a mode number dependent cut-off radius beyond which theycannot propagate.

Shown in FIG. 12 are the SPP dispersion relationships calculated fromEq. (2) using Drude parameters for Au, and air for different cone radii.The resulting continuous transformation of SPP's propagating on atapered waveguide presents a highly efficient antenna concept. It can beseen that for all radii the SPP wavevector kSPP is larger than that oflight in free space, with k-vectors increasing with decreasing radius.FIG. 13 shows the increase in n_(eff)(r), labeled 100, for the m=0 modecalculated from Eq. (2) for the case of λ0=633 nm and a cone half-angleof θ=3.5° as an example. Associated with the divergence of n_(eff)(r) isa decrease in the group velocity vg=dw/dk, labeled 102. This gives riseto a decrease in λSPP, as seen in the spatial evolution of the surfaceelectric field down the cone, calculated from Stockman, M. I. Phys. Rev.Lett., 93, 137404 (2004), and taking into account propagation damping,labeled 104. Concomitantly, the index of refraction increase leads to adecrease of the spatial extent of the evanescent SPP field into thedielectric medium given by 1/Im (2kSPP, r) labeled 106. This increasesthe spatial confinement of the mode on the waveguide. The combination ofthese effects leads to the concentration of the electric field into thecone apex, as seen in the rising electric field amplitude. (see BerwegerII)

In the experimental implementation of 3D SPP nanofocusing,considerations arise in terms of the choice of waveguide fabricationmethod and the SPP launching mechanism used. Conical tips with smoothsurfaces and uniform taper angles, such as those used in scanning probeapplications, can be obtained by electrochemical etching from bulk wire.Alternatively, tips grown with electron-beam induced chemical vapordeposition are also suitable, but are more time- and equipment-intensiveto fabricate. Other approaches, for example utilizing template-strippingprocedures, have also been demonstrated. Gold (Au) tips are preferablyused, primarily due to the ambient stability of the metal and the easeof tip fabrication, though additional benefits could be gained throughthe use of Silver (Ag) as the waveguiding material. In Au, SPPpropagation is associated with high losses near the resonant interbandtransition, which become especially pronounced for photon energies above˜2 eV. Using Ag as a waveguide material allows for low propagationlosses and therefore higher nanofocusing efficiencies, butelectrochemical etching methods provide challenges.

In order to overcome the momentum mismatch discussed above and launchSPP's onto these tips, the traditional methods of grating-coupling orattenuated total internal reflection (ATR) can be used, as well asphotonic crystal elements or coupling from dielectric waveguides. Theuse of prism-based ATR coupling elements is difficult due to thegeometric constraints of micron-scale SPP waveguides, and despite beingpossible, has thus far remained impractical. While end-on couplingbetween dielectric and plasmonic waveguides can in principle be highlyefficient, this remains difficult in 3D structures due to the highpositioning accuracy necessary at visible frequencies, and alternativemethods such as coupling to SPP modes of a metallic cladding on atapered waveguide suffer from poor mode overlap. In contrast to ATRgeometries, grating coupling elements can readily be fabricated viafocused ion beam milling (FIB) onto the shaft of nanofocusing waveguidesand large theoretical coupling efficiencies are possible. An alternativeapproach has been to use a photonic crystal cavity to localize andcouple SPPs onto the base of tip, which allows for a transmission-typegeometry with facile alignment, but this suffers from residualhole-array transmission and far-field radiation superimposed with theapex field. The spatial confinement of optical fields achieved in suchan antenna was established in a scanning probe geometry (as shown inFIG. 14—for the prior art shear force microscopy case) by using anultrasharp step edge as an effective point scatterer and measuring thespatial extent of the apex field. From the scattering signal, a size ofthe nanofocus of ˜22±5 nm was determined.

Measurements of emitted intensity indicate that 2-4% of the lightinitially incident on the grating within the coupling bandwidth isre-emitted at the apex of the tip. Despite the high loss during gratingcoupling and propagation, confined to a (20 nm) volume, thisnevertheless represents a power density two orders of magnitude higherthan a diffraction-limited focus with the same initial intensity.

The ability to generate a nano-localized optical excitation at the endof a scanning probe tip with high nanofocusing efficiency holdssignificant promise for background free near-field spectroscopy. Inconventional s-SNOM implementations, the far-field focus, which is usedto excite the scanning probe tip, will often generate a largebackground, so that demodulation techniques are necessary in order toextract the near-field signal, especially in linear s-SNOM. (seeBerweger II)

Applications

As further outlined in Berweger II, background-free TERS is expected tobe particularly useful for the study of crystalline systems, where thefar-field Raman background is strong due to the extended material volumecompared to a molecular monolayer. Remote-excitation TERS according tothe preferred embodiments also holds promise for the full realization ofthe capabilities of nano-Raman spectroscopy for the study of, e.g.,strain and domain formation in crystalline or molecular nanocompositematerials on nanometer length scales.

Additionally, spatially resolved imaging using elastic fight scatteringwith no background can be achieved. This plasmonic nanofocusing s-SNOMhas applications similar to, and in many cases complementary to,conventional s-SNOM. In particular, it will improve the capabilities ofelastic fight scattering and vibrational IR nano-spectroscopy to image,for example, domains in correlated electron materials and other spatialinhomogeneities such as phase separation in polymer blends.

Ultrafast spectroscopy with femtosecond pulses has enabled the study ofmatter on the characteristic time scales of the elementary electronicand vibrational excitations. Shaping the amplitude and phase ofultrafast pulses allows for coherent control of quantum systems.Extension of these techniques to the nanoscale through plasmonicnanofocusing will allow the all-optical study of the elementaryexcitations of matter not only on their characteristic time, but alsolength scales given by electronic and lattice correlations, as shown inFIG. 15. The approach would complement the related emerging capabilitiesof ultrafast electron and x-ray imaging. As an all-optical technique, bycoupling directly to the electronic and nuclear degrees of freedom, itallows for the study and quantum coherent control of dynamicinteractions in molecular and solid matter on the nanoscale, and at timescales down to the single cycle limit. The use of nanofocusing of SPP'sinto the nanometer apex of noble metal tips is compatible withimplementations of a wide range of spectroscopic techniques. In additionto the improvements gained in TERS and elastic scattering measurementsfrom efficient background suppression, the high nanofocusing efficiencyand broad wavelength range allows the extension of the technique inprinciple to any wavemixing process, including multi-dimensionalspectroscopies. Furthermore, the high nanoscale fields that can begenerated at the tip apex are sufficient for electron emission viaeither multiphoton or optical tunneling processes. Strong associatedfield gradients and optical forces can be used for nanomanipulation andtrapping. The control and localization of pulses could also be utilizedfor higher harmonic generation (HHG) using only high repetition rateTi:Sa oscillator pulses. (see Berweger II)

Adiabatic tip-plasmon focusing can readily be extended to a range ofwavelengths by adjusting the grating parameter and considering coneangle, SPP damping, and other wavelength-related parametersappropriately. Performing TERS at long wavelengths is desirable withreduced fluorescence and enhanced sample transparency in biologicalmedia. Nanofocusing is possible over a broad spectral range, determinedby the wavelength-dependent SPP damping and related taper angle. Forgold (Au) tips, guided by established etching procedures and ambientstability, the SPP propagation length given by L=1/Im(kSPP), resultingfrom material damping, imposes a wavelength of λ≅600 nm as the lowerpractical limit (L≅9 μm); for silver tips, wavelengths down to λ≅400 nmcan be used (L≅10 μm for λ=400 nm). (see Berweger II)

In spectroscopy in general, and particularly for nanospectroscopy, thefundamental limit to sensitivity and emitter localization is oftendetermined by the contrast (near-field to background signal ratio),where the background level is frequently difficult to remove. In TERS,the near-to far-field signal contrast is directly related to the degreeand spatial extent of the near-field enhancement region relative to thefar-field focus size. A high contrast and suppression of the far-fieldbackground can be achieved in various TERS geometries with a combinationof large field enhancement and the use of high-NA optics, and can befurther benefited by spatially dispersed sample material, yet is stillpractically constrained by the diffraction limit. In grating-couplingTERS, the far-field signal is negligible and principally of a differentnature, as a consequence of the nonlocal excitation with no directfar-field sample irradiation and the effective suppression of anyresidual grating-scattered light via confocal spatial filtering. It isof note that the unique background suppression mechanism ofgrating-coupling TERS is effective without the need for high-NA opticsor large field-enhancement values.

Further improvement in the technique can be achieved by refining theexcitation and detection geometry, particularly the use of confocalfiltering in combination with a high-NA objective in an axial detectionscheme, as well as optimized grating parameters, incident angle, andapex-grating separation distance.

Also, conventional cantilever AFM tips can be used after metal coating.Through the use of compatible high-NA axial detection scheme fortransparent samples, improved signal collection can be achieved.Moreover, the application in liquid environments typically requires themodification of grating parameters, and/or incident angle, for efficientgrating-coupling given that SPPs propagating across a liquid-airinterface can possibly experience scattering and reflection losses.Efficient adiabatic plasmon nanofocusing in monolithic Au scanning probetips for TERS allows for the extension of TERS in the near-IR, asdemonstrated for λ=800 nm. The spatial separation of the far-fieldgrating-coupling of the radiation and the propagation-induced near-fieldapex localization allows for an improvement in near to far-fieldcontrast in TERS with effective suppression of the far-field backgroundwith multiple excitation wavelengths. Moreover, this capability fornonlocal generation of a nanoscale excitation source with truenanofocusing efficiency and intrinsic background suppression facilitatesa wide range of spectroscopic techniques. (see Berweger II)

Although the best mode contemplated by the inventors of carrying out thepresent invention is disclosed above, practice of the above invention isnot limited thereto. It will be manifest that various additions,modifications and rearrangements of the features of the presentinvention may be made without deviating from the spirit and the scope ofthe underlying inventive concept.

What is claimed is:
 1. An SPM including: a probe including a cantileverand a tip, the tip positioned at a distal end of the cantilever andincluding a) a shaft and b) an apex positioned adjacent to a sample; areceiving element supported by the shaft of the tip; and a source ofelectromagnetic wave excitation that directs electromagnetic wavestoward the receiving element, the electromagnetic waves being coupled tothe apex via the receiving element, and wherein the coupledelectromagnetic waves at the apex yield locally enhanced, backgroundfree spectroscopic signal that interacts with the sample.
 2. The SPM ofclaim 1, further comprising a controller that maintains a separationbetween the apex and the sample greater than zero nanometers and lessthan 100 nm during electromagnetic wave excitation.
 3. The SPM of claim2, wherein the separation is maintained at less than 10 nm duringelectromagnetic wave excitation.
 4. The SPM of claim 2, wherein thecontroller maintains the separation using o torsional resonance mode (TRMode) feedback and contact mode feedback.
 5. The SPM of claim 2, whereinthe controller maintains the separation at less than 5 nm using tappingmode, wherein a tapping mode setpoint amplitude is between about 0.1 nmand 10 nm.
 6. The SPM of claim 1, wherein the shaft has a continuoussurface around its entire periphery.
 7. The SPM of claim 6, wherein thecontinuous surface is at least one of a conical surface and anelliptical surface.
 8. The SPM of claim 1, wherein the receiving elementis supported by the tip and is one of a surface grating, a prism, aphotonic crystal, a waveguide, and an optical antenna.
 9. The SPM ofclaim 1, wherein the apex comprises a conductive metal, and has a radiusbetween about 1 and 100 nm.
 10. A method for optically measuring aphysical property of a sample, the method including: providing ascanning probe microscope (SPM) including a probe having a cantileverand a tip supported at about a distal end of the cantilever, the tipincluding a shaft and an apex; providing a nanostructure at the apex ofthe tip; illuminating and thereby optically exciting the nanostructureso as to produce a locally enhanced spectroscopic signal that interactswith the sample; and measuring a property of the sample based on theinteraction.
 11. The method of claim 10, wherein the optically excitingstep is performed off-resonance.
 12. The method of claim 10, wherein thenanostructure is at least one of a group including a quantum dot, amolecule and a sharp tip with a radius less than about 1 nm.